Callback split: read_cpuacct_procs

The callback "read_cpuacct_procs" used to read the ".stat" file by itself.

This commit splits this callback into two parts:
* read_cpuacct_procs: creates the full path of the file to read
* process_cgroup_file: reads the given file and processes the metrics

It will allow people to add other metrics than CPU accounting. For example:
* disk I/O
* memory

More pull requests coming soon...